1. Field of the Invention
The present invention relates to a printing apparatus and method for printing images by forming ink dots, and more specifically, to a printing apparatus and method for printing letters or images by applying inks on the basis of print data to form ink dots on a printing medium.
2. Description of the Related Art
Some printing apparatuses such as printers, copiers, and facsimile machines print images by forming a dot pattern on a printing medium, for example, paper or a thin plastic film, on the basis of image information. These printing apparatus use various printing systems such as an ink jet type, a wire dot type, a thermal type, and a laser beam type. In recent years, it has been desired to increase the print speed of the printing apparatus, improve the quality of images (resolution) formed by the printing apparatus, and reduce noise from the printing apparatus.
The ink jet type printing apparatus (ink jet printing apparatus) meet these requirements. The ink jet printing apparatus print an image by attaching ink (print liquid) droplets ejected from ejection openings in a print head, to a printing medium. Further, the ink jet printing apparatus can print an image on a printing medium in a non-contact manner. Accordingly, it can stably print images on various printing media. Furthermore, the ink jet printing apparatus enables multiple color inks to be easily ejected from the print head. It is thus utilized to print various types of data ranging from texts (letters) formed using a black ink to colored images formed using multiple color inks. In this case, the top priority is given to the print speed for the printing of texts formed using the black ink and to the granular impression and gradation of images for the printing of colored images formed using the multiple color inks. Thus, some ink jet printing apparatuses print a text by ejecting a relatively large amount of black ink on the basis of binary data (indicating whether or not the ink is to be ejected), while printing a colored image by ejecting relatively small amounts of multiple color inks other than the black ink, such as a cyan, magenta, and yellow inks (hereinafter referred to as “color inks”) on the basis of multivalued data.
For such ink jet printing apparatuses, a method has been proposed which uses a print density lower than the other methods in order to reduce ink consumption and improve the print speed.
For example, Japanese Patent Application Laid-open No. 6-139348(1994) proposes such an image processing method. With this method, image data of print resolution 600 dpi (dots/inch) (600 dpi for each print area) is first divided into blocks each composed of four (2×2) print areas. Then, if a print dot is present in any block, the print resolution of the image data is converted into a lower one so that one dot is printed in one print area of print resolution 300 dpi. Japanese Patent Application Laid-open No. 10-024567(1998) proposes a method of expanding each pixel unit of a print signal to bit map data that can be formed by a plurality of dots and thinning the dots so that one pixel contains at least one dot.
It is also contemplated that if normal print data is processed at a relatively high resolution of 600 dpi, the time required for processing may be reduced by using a relatively low resolution of 300 dpi. Moreover, if a bit map is used to convert binary print data of 300 dpi so as to increase the resolution to 600 dpi, the ink consumption may be reduced or the print speed may be increased by performing control such that only two dots are printed in a unit block composed of four (2×2) print areas each of 600 dpi.
However, if printing is executed by thinning print data (thinned printing), it is difficult to increase both the print grade of texts (letters) printed using the black ink and the print grade of colored images printed using the color inks.
For example, as described in Japanese Patent Application Laid-open No. 6-139348(1994), printing can be accomplished without partly missing thin lines of one dot width. However, for printing of images, if one or more dots are present in a unit block composed of four (2×2) print areas each having a print resolution of 600 dpi, the print density may disadvantageously be uniform, thus impairing gradation.
For example, as shown in FIG. 15A, a solid image (50% duty solid image) having a print duty 50% at a print resolution of 600 dpi is assumed. In the solid image, printing is uniform all over the print areas. If this image is printed in association with binary data with a normal resolution of 600 dpi, that is, binary data (“1” or “0”) indicating whether or not ink is to be ejected into one print area of 600 dpi, the image is printed so as to have a duty of 50% as shown in FIG. 15B. Thus, 2 dots are formed in four (2×2) print areas each of 600 dpi. If this image is subjected to the image processing described in Japanese Patent Application Laid-open No. 6-139348, one dot is formed in one print area of 300 dpi to provide a solid image of print duty 100% as shown in FIG. 15C.
Now, as shown in FIG. 16A, a solid image (25% duty solid image) having a print duty 25% at a print resolution of 600 dpi is assumed. If this image is printed in association with binary data with the normal resolution of 600 dpi, the print duty will be 25% as shown in FIG. 16B. If this image is subjected to the image processing described in Japanese Patent Application Laid-open No. 6-139348(1994) as in the case of FIG. 15C, one dot is formed in one print area of 300 dpi as shown in FIG. 16C. Consequently, a solid image of print duty 100% is formed. This image cannot be distinguished from the image shown in FIG. 15C. Thus, the image processing described in Japanese Patent Application Laid-open No. 6-139348(1994) may significantly impair the gradation of images.
On the other hand, Japanese Patent Application Laid-open No. 10-024567(1998) is premised on the printing of one pixel using a plurality of dots. Accordingly, this method cannot be implemented if up to one dot is printed in one pixel. Further, if one dot is printed using a plurality of dots, the print speed decreases as described in Japanese Patent Application Laid-open No. 10-024567(1998). Moreover, if one pixel is printed using N (an integer of at least 2) dots, a possible thinning rate is 1/N. Consequently, the resulting print grade is insufficient.
During the thinned printing, in which a certain number of dots are thinned, if the print solution is reduced, the following problems may occur.
For example, as shown in FIG. 17A, a solid image (25% duty solid image) having a print duty of 25% at a print resolution of 600 dpi is assumed. If this image is printed in association with binary data with the normal resolution of 600 dpi, the print duty will be 25% as shown in FIG. 17B. Further, if the image shown in FIG. 17A is printed in association with binary data of 300 dpi, the dots are arranged as shown in FIG. 17C. If the image of 300 dpi, shown in FIG. 17C, is expanded so as to have a higher resolution of 600 dpi, that is, the binary data in a print area of 300 dpi is expanded into four (2×2) print areas of 600 dpi, the image shown in FIG. 17D is obtained when the dots are thinned by 50%. That is, the image shown in FIG. 17D is printed by executing thinning by 50% to form two dots in four (2×2) print areas, instead of four dots otherwise formed in the same print areas.
The image shown in FIG. 17D is obtained by thinning the dots in FIG. 17B by 50%. It is thus possible to reduce the ink consumption and improve the print speed. However, within four (2×2) print areas, two dots are printed close to each other so as to make a pair, thus impairing the uniformity of dot arrangement. If the print resolution and then the number of dots used to form one pixel are reduced in order to execute the thinning, the print grade of images may lower.
Thus, with the conventional printing apparatuses, if the thinned printing is executed to reduce the amount of ink for printing, the image grade may lower. Further, to prevent the degradation of images, it is necessary to use a plurality of dots to print one pixel for the normal printing, while using a reduced number of dots to print one pixel for the thinned printing. In this case, the normal print speed may decrease.